1. Field of the Invention
The present invention is directed towards a power source and, more particularly, to a resonant current-driven power source. In the presently preferred embodiment, the power source is constructed as a DC to DC converter regulator. The power source can also be constructed as a DC to AC inverter.
2. Description of the Prior Art
Converters of the prior art fall into two primary categories: voltage-driven converters and current-driven converters. A typical voltage-driven converter is illustrated in FIG. 1. As shown therein, the converter comprises four switching transistors Q1-Q4 which are connected between a source voltage Es and the primary winding W1 of a transformer T. A control circuit (not shown) applies inverse square waves to transistors Q1-Q4 so as to operate the transistors in a saturated square wave power "chopper" mode. In this mode, transistor pairs Q1, Q4 and Q2, Q3 are alternately turned on and off so as to induce an AC voltage across the primary winding of transformer T. Particularly, transistors Q1 and Q4 are initially driven into saturation while transistors Q2 and Q3 are turned off so that a current flows through the primary winding W1 of transformer T in the positive direction of current i. Thereafter, transistors Q1 and Q4 are turned off and transistors Q2 and Q3 are driven into saturation with the result that a current flowing in the opposite direction from that of current i will flow through the primary winding W1 of transformer T. The effect of the foregoing is to convert the DC supply voltage Es to an AC voltage ep with a peak-to-peak value of approximately 2Es. The AC voltage across the primary winding of transformer T induces a stepped up or stepped down voltage (depending upon the turns ratio of tranformer T) across the secondary winding W2 of transformer T. This induced voltage is applied across the full wave bridge rectifier BR so as to charge capacitor C to the desired output lever Eo.
Since there is no resistance in the charging path to capacitor C, capacitor C acts as a peak detector and stores a voltage Eo which is determined only by the voltage ep across the primary winding W1 of transformer T and the turns ratio of transformer T. As such, the output voltage Eo of the voltage converter of FIG. 1 is related to the input supply voltage Es by a constant ratio. This makes it impossible to electronically change the input to output voltage ratio and prevents the voltage-driven converter circuit from varying the output voltage (for a given source voltage) from compensating for variations in line voltage, or for variations due to load regulation. As such, the voltage-driven converter cannot operate as a regulator.
As shown in FIG. 1, an equivalent capacitance Cs appears across the primary winding W1 of transformer T. This capacitor represents both the stray capacitance across the primary winding W1 and the reflected stray capacitance across the secondary winding W2 of transformer T. The alternating square wave AC voltage appearing across points A and B in FIG. 1 cause an AC current i to flow in the capacitance Cs which results in a significant volt-ampere load which must be dissipated in the switching transistors Q1-Q4. This loss is one of the major factors limiting the efficiency and operating frequency of a voltage-driven DC/DC converter, especially for high voltage converters where Cs tends to be large.
Due to the practical constraints in the timing of the switching wave forms applied to transistors Q1-Q4 and possible variations in the switching times of the transistors, the voltage between points A and B will have a small but significant DC component. Since there is minimal DC resistance across the primary winding W1 of transformer T, even a low DC voltage across points A and B can result in a high DC current to flow through the primary winding W1 of transformer T. This current can easily saturate transformer T and thereby degrade the operation of the converter circuit. To avoid this problem, prior art regulators typically place a capacitor C' in series with the primary winding W1 of transformer T. While this capacitor prevents saturation of the transformer, it increases the size and cost of the converter circuit.
In an effort to overcome some of the shortcomings of the voltage-driven converter, the prior art has developed various current-driven inverters, a typical example of which is illustrated in FIG. 2. In this circuit, the charging capacitor C has been removed from the diode bridge BR and placed across the output load Ro. An inductor L has been placed in series with the capacitor C and forms an integrating circuit in the output path of diode bridge BR. As such, the magnitude of the output voltage Eo can be modified by modifying the duration of the circuit pulses produced by diode bridge Br. The duration of the pulses can, in turn, be controlled by controlling the duty cycle of the driving pulses applied to transistors Q1-Q4. In this manner, the current-driven converter can operate as a regulator.
The primary drawbacks of the prior art current-driven DC /DC converter are as follows. Initially, the integrating inductor L must be large (and therefore expensive) since it must accommodate all of the DC current and store enough energy at all operating currents to maintain continuous output current flow. Furthermore, its resistance must be adequately low to avoid efficiency degrading losses and thermal problems.
In addition to the foregoing, the efficiency reducing volt-ampere losses across equivalent capacitor Cs are essentially the same as those of the voltage-driven converter of FIG. 1 and a filter capacitor C' must again be placed in series with the primary winding W1 of transformer T to protect the transformer from the DC voltage across points A and B.
In most practical applications, the converter circuit of FIG. 2 is operated between 20% and 100% of the rated load. This circuit is not normally operated below 20% of the rated load since this would require a larger and more expensive inductor to store the necessary energy at the lower currents. In order to avoid the need for a larger inductor, a bleeder resistor is sometimes connected across the capacitor C to provide the minimum current of 20% of rated load. This resistor, however, causes undesirable losses in the circuit.